1. Technical Field
The present invention relates to control systems for the fuel circuit from a fuel reservoir to the injectors of a gas turbine engine. Such circuits typically comprise in series a low pressure pump connected to the reservoir, a high pressure pump, a throttle valve and a pressuring valve connected to the injectors. A pressure drop detector is connected across the throttle valve and a by-pass valve is connected between the outlet and the inlet of the high pressure pump so that, in response to the variations of the pressure drop across the throttle valve the by-pass valve is closed or opened more or less in order to adjust the recirculated fuel flow from the outlet of the high pressure pump back to the inlet and, maintain constant that pressure drop.
Military standards require that engine designers install a stop valve in the fuel supply circuit for new jet engines. Such mechanical stop valves are usually manually operated from the pilot's control column in order to allow interruption of the fuel supply to the injectors or by an auxiliary control lever when a thrust reverser selection is embodied.
2. Prior Art
A mechanical control system is described in French patent specification No. 2 180 483. This system is diagrammatically illustrated in FIG. 1 of the drawings. Fuel from the reservoir which is at the aircraft pressure (PCA) is delivered by a low pressure pump 1 at a low pressure BP to the inlet of a high pressure pump 2 which supplies the injectors 5 of a gas turbine engine via a throttle valve 3 and a stop and pressurizing valve 6. The fuel output from the high pressure pump at pressure PAM is passed to a throttle valve which is controlled at the control column and which directs fuel both to the start-up injectors 4 and via the stop and pressurizing valve 6 to the main injectors 5. The stop and pressurizing valve 6 only permits flow of fuel to the main injectors when combustion has been properly initiated in the combustion chamber. A purging system 8 is also connected into the fuel circuit to permit purging of the injectors when the engine is stopped. A pressure drop detector 9 is connected between the inlet and the outlet of the throttle valve 3 and has an output which is connected to a by-pass input of a control valve 10. The control valve 10 is connected between the outlet and the inlet of the high pressure pump and by-passes a part of the outlet flow from the high pressure pump back to the pump inlet.
The mechanical control of the described arrangement is effected by a three position valve actuated manually by the pilot. In the start-up phase, the valve is positioned so that it controls the stop and pressurizing valve 6 in such a manner that the pump pressure acts against the spring force in a secondary valve which prevents supply of fuel to the main injectors until the pump has reached its correct operational pressure rating. Meanwhile, fuel is being supplied by the high pressure pump 2 to the start-up injectors 4.
In order to stop the jet engine, the three position valve 7 is positioned so as to by-pass the fuel output from the high pressure pump back to the inlet of that pump.
The operation of this system is more fully described in the French patent specification No. 2 180 483.